storage: allow to choose rebalancer location

[Gerold103]

Currently people struggle to find the rebalancer location because it is unclear where it is spawned automatically (on the master with minimal uuid).

The patch allows to choose in the configuration where the rebalancer should work and whether it should work at all.

Closes #278

@TarantoolBot document Title: vshard: cfg to choose rebalancer mode and location

There are 2 new config options:

• Root option 'rebalancer_mode'. It can have one of the following values:

  • 'auto' - works like now. The rebalancer is selected as the master with the minimal UUID;
  • 'off' - rebalancer won't be active at all;
  • 'manual' - need to manually mark the node where the rebalancer should be spawned. The selection happens in the config.

  Default is 'auto'.

• Instance option 'rebalancer'. It can be true or false. At most one instance in the config can have it set to true. If the rebalancer mode is 'manual' and none of the instances have this flag true, then the rebalancer won't be started anywhere. This option is ignored when the mode is not 'manual'. Default is false.

[Totktonada]

I would like to configure a replicaset to work in the rebalancer role, that means that a master instance in this replicaset runs the rebalancer.

[Gerold103]

@Totktonada this is not possible until master=auto is implemented for storages. In the current version you can set rebalancer flag together with master flag.

[Serpentian]

@Gerold103, the PR should be probably closed